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      <div class="titlepage">
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          <div>
            <h2 class="title"><a id="repapp"></a>Chapter 3. The DB Replication Manager</h2>
          </div>
        </div>
      </div>
      <div class="toc">
        <p>
          <b>Table of Contents</b>
        </p>
        <dl>
          <dt>
            <span class="sect1">
              <a href="repapp.html#rep_init_code">
        Starting and Stopping Replication
    </a>
            </span>
          </dt>
          <dd>
            <dl>
              <dt>
                <span class="sect2">
                  <a href="repapp.html#election_flags">Managing Election Policies</a>
                </span>
              </dt>
              <dt>
                <span class="sect2">
                  <a href="repapp.html#thread_count">Selecting the Number of Threads</a>
                </span>
              </dt>
            </dl>
          </dd>
          <dt>
            <span class="sect1">
              <a href="repmgr_init_example_c.html">Adding the Replication Manager to
                
                
                <span>SimpleTxn</span>
        </a>
            </span>
          </dt>
          <dt>
            <span class="sect1">
              <a href="fwrkpermmessage.html">Replication Manager Permanent Message Handling</a>
            </span>
          </dt>
          <dd>
            <dl>
              <dt>
                <span class="sect2">
                  <a href="fwrkpermmessage.html#fmwrkpermpolicy">Identifying Permanent Message Policies</a>
                </span>
              </dt>
              <dt>
                <span class="sect2">
                  <a href="fwrkpermmessage.html#fmwrkpermtimeout">Setting the Permanent Message Timeout</a>
                </span>
              </dt>
              <dt>
                <span class="sect2">
                  <a href="fwrkpermmessage.html#perm2fmwrkexample">Adding a Permanent Message Policy to 
                            
                            
                            <span>RepQuoteExampleGSG</span>
                    </a>
                </span>
              </dt>
            </dl>
          </dd>
          <dt>
            <span class="sect1">
              <a href="electiontimes.html">Managing Election Times</a>
            </span>
          </dt>
          <dd>
            <dl>
              <dt>
                <span class="sect2">
                  <a href="electiontimes.html#electiontimeout">Managing Election Timeouts</a>
                </span>
              </dt>
              <dt>
                <span class="sect2">
                  <a href="electiontimes.html#electretrytime">Managing Election Retry Times</a>
                </span>
              </dt>
            </dl>
          </dd>
          <dt>
            <span class="sect1">
              <a href="fmwrkconnectretry.html">Managing Connection Retries</a>
            </span>
          </dt>
          <dt>
            <span class="sect1">
              <a href="heartbeats.html">Managing Heartbeats</a>
            </span>
          </dt>
        </dl>
      </div>
      <p>
        The easiest way to add replication to your transactional
        application is to use the Replication Manager. The Replication Manager provides a comprehensive
        communications layer that enables replication. For a brief listing
        of the Replication Manager's feature set, see 
        <a class="xref" href="apioverview.html#repframeworkoverview" title="Replication Manager Overview">Replication Manager Overview</a>.
    </p>
      <p>
            To use the Replication Manager, you make use of special methods off the
            <code class="classname">Environment</code> and
            <code class="classname">EnvironmentConfig</code> classes, as well as
            the <code class="classname">ReplicationManagerSiteConfig</code> class.
            You also use a series of related classes to perform your
            implementation. For example, in order to detect whether your
            code is running as a master or a replica, you must implement
            <code class="classname">com.sleepycat.db.EventHandler</code>.
            (see <a class="xref" href="fwrkmasterreplica.html#determinestate" title="Determining State">Determining State</a>).
            That is:
    </p>
      <div class="orderedlist">
        <ol type="1">
          <li>
            <p>
                Create an environment handle as normal.
            </p>
          </li>
          <li>
            <p>
                Configure your environment handle as needed (e.g. set the
                error file and error prefix values, if desired).
            </p>
          </li>
          <li>
            <p>
                Use the Replication Manager replication classes and methods to configure
                the Replication Manager.  Using these classes and methods causes DB
                to know that you are using the Replication Manager.
            </p>
            <p>
                Configuring the Replication Manager entails setting the replication
                environment's priority, setting the TCP/IP address that
                this replication environment will use for incoming
                replication messages, identifying TCP/IP addresses of other
                replication environments, and so forth.  These actions are
                discussed throughout the remainder of this chapter.
            </p>
          </li>
          <li>
            <p>
                Open your environment handle. When you do this, be sure to
                specify 

                
                
                <span>
                    <code class="methodname">EnvironmentConfig.setInitializeReplication()</code>
                    when you configure your environment handle. This is in
                    addition to the configuration that you would normally
                    use for a transactional application.  This causes
                    replication to be initialized for the application.
                </span>
            </p>
          </li>
          <li>
            <p>
                Start replication by calling
                
                
                <span><code class="methodname">Environment.replicationManagerStart()</code>.</span>
            </p>
          </li>
          <li>
            <p>
                Open your databases as needed. Masters must open their databases
                for read and write activity. Replicas can open their databases
                for read-only activity, but doing so means they must
                re-open the databases if the replica ever becomes a master.
                Either way, replicas should never attempt to write to the
                database(s) directly.
            </p>
          </li>
        </ol>
      </div>
      <p>
        When you are ready to shut down your application: 
    </p>
      <div class="orderedlist">
        <ol type="1">
          <li>
            <p>
                Close your databases
            </p>
          </li>
          <li>
            <p>
                Close your environment. This causes replication to stop as
                well.
            </p>
          </li>
        </ol>
      </div>
      <div class="note" style="margin-left: 0.5in; margin-right: 0.5in;">
        <h3 class="title">Note</h3>
        <p>
            Before you can use the Replication Manager, you may have to enable it
            in your DB library. This is
            <span class="emphasis"><em>not</em></span> a requirement for Microsoft
            Windows systems, or Unix systems that use pthread
            mutexes by default. Other systems, notably BSD and
            BSD-derived systems (such as Mac OS X), must enable the
            Replication Manager when you configure the DB build.
        </p>
        <p>
            You do this by <span class="emphasis"><em>not</em></span> disabling
            replication and by configuring the library with POSIX
            threads support. In other words, replication must be
            turned on in the build (it is by default), and POSIX
            thread support must be enabled if it is not already by
            default.  To do this, use the
            <code class="literal">--enable-pthread_api</code> switch on the
            configure script.
        </p>
        <p>
            For example:
        </p>
        <pre class="programlisting">../dist/configure --enable-pthread-api</pre>
      </div>
      <div class="sect1" lang="en" xml:lang="en">
        <div class="titlepage">
          <div>
            <div>
              <h2 class="title" style="clear: both"><a id="rep_init_code"></a>
        Starting and Stopping Replication
    </h2>
            </div>
          </div>
        </div>
        <div class="toc">
          <dl>
            <dt>
              <span class="sect2">
                <a href="repapp.html#election_flags">Managing Election Policies</a>
              </span>
            </dt>
            <dt>
              <span class="sect2">
                <a href="repapp.html#thread_count">Selecting the Number of Threads</a>
              </span>
            </dt>
          </dl>
        </div>
        <p>
        As described above, you introduce replication to an application by
        starting with a transactional application, performing some basic
        replication configuration, and then starting replication using 
        
        
        <span><code class="methodname">Environment.replicationManagerStart()</code>.</span>
    </p>
        <p>
        You stop replication by closing your environment
        cleanly in the same way you would for any DB application.
    </p>
        <p>
        For example, the following code fragment initializes, then
        stops and starts replication. Note that other replication
        activities are omitted for brevity.
    </p>
        <div class="note" style="margin-left: 0.5in; margin-right: 0.5in;">
          <h3 class="title">Note</h3>
          <p>
            Note that the following code fragment would be part of a larger
            class that must implement
            <code class="classname">com.sleepycat.db.EventHandler</code>. This
            class is used to track state changes between master and
            replica. We put off that implementation for the moment, but the
            point remains that the following code fragment would be
            contained in a method or two that you would include in your 
            <code class="classname">com.sleepycat.db.EventHandler</code> 
            implementation. 
        </p>
        </div>
        <pre class="programlisting">
import com.sleepycat.db.Environment;
import com.sleepycat.db.EnvironmentConfig;
import com.sleepycat.db.ReplicationHostAddress;
import com.sleepycat.db.ReplicationManagerStartPolicy;
import com.sleepycat.db.ReplicationManagerSiteConfig;

...
    String progname = "example_replication";
    String envHome = "TESTDIR";
    int cachesize = 10 * 1024 * 1024;

    Environment dbenv;
    String listenHost = "mymachine.sleepycat.com";
    String otherHost = "anothermachine.sleepycat.com";
    int listenPort = 8080;
    int otherPort = 8081;


try {
    // Configure the environment handle
    EnvironmentConfig envConfig = new EnvironmentConfig();
    envConfig.setErrorStream(System.err);
    envConfig.setErrorPrefix(progname);
    envConfig.setCacheSize(cachesize);
    envConfig.setTxnNoSync(true);

    // Identify the local replication site. This is the local hostname
    // and  port that this replication participant will use to receive
    // incoming replication messages. Note that this can be
    // performed only once for the application. It is required.
    ReplicationManagerSiteConfig thisHostConfig = 
        new ReplicationManagerSiteConfig(listenHost, listenPort);
    thisHostConfig.setGroupCreator(creator);
    thisHostConfig.setLocalSite(true)
    envConfig.addReplicationManagerSite(thisHostConfig);

    // Set this application's priority. This is used for elections.
    // 
    // Set this number to a positive integer, or 0 if you do not want
    // this site to be able to become a master.
    envConfig.setReplicationPriority(100);

    // Add a site to the list of replication environments known to 
    // this application.
    ReplicationManagerSiteConfig remoteSiteConfig =
        new ReplicationManagerSiteConfig(otherHost, otherPort);
    remoteSiteConfig.setBootstrapHelper(true);
    envConfig.addReplicationManagerSite(remoteSiteConfig);

    // Configure the environment's subsystems. Note that we initialize
    // replication. This is required.
    envConfig.setAllowCreate(true);
    envConfig.setRunRecovery(true);
    envConfig.setThreaded(true);
    envConfig.setInitializeReplication(true);
    envConfig.setInitializeLocking(true);
    envConfig.setInitializeLogging(true);
    envConfig.setInitializeCache(true);
    envConfig.setTransactional(true);

    // Missing from this is where we set the event handle and the
    // acknowledgement policy. We discuss these things later in this 
    // book. 

    // Open our environment handle.
    try {
        dbenv = new Environment(envHome, envConfig);
    } catch(FileNotFoundException e) {
        System.err.println("FileNotFound exception: " + e.toString());
        System.err.println(
            "Ensure that the environment directory is pre-created.");
    }

    // Start the replication manager such that it has three threads.
    dbenv.replicationManagerStart(3,
        ReplicationManagerStartPolicy.REP_ELECTION);

    //////////////////////////////////////////////////
    // All other application code goes here, including
    // database opens.
    //////////////////////////////////////////////////

} catch (DatabaseException dbe) {
    // Error handling goes here
}

// Close out your application here.
try {
    // Make sure all your databases are closed.

    // Closing your environment stops replication.
   dbenv.close();
} catch (DatabaseException dbe) {
    // Error handling here.
}

// All done.  </pre>
        <div class="sect2" lang="en" xml:lang="en">
          <div class="titlepage">
            <div>
              <div>
                <h3 class="title"><a id="election_flags"></a>Managing Election Policies</h3>
              </div>
            </div>
          </div>
          <p>
                Before continuing, it is worth taking a look at the

                

                <span>
                        startup election options that you can set for
                        replication. You set these using the
                        <code class="classname">ReplicationManagerStartPolicy</code> 
                        class that you pass to the
                        <code class="methodname">Environment.replicationManagerStart()</code>
                        method.
                        
                </span>
            </p>
          <p>
                    In the previous example, we specified 
                        
                        <code class="literal">ReplicationManagerStartPolicy.REP_ELECTION</code>
                    when we started replication. This causes the
                    application to try to find a master upon startup. If it
                    cannot, it calls for an election. In the event an
                    election is held, the environment receiving the most number of
                    votes will become the master.
            </p>
          <p>
                    There's some important points to make here:
            </p>
          <div class="itemizedlist">
            <ul type="disc">
              <li>
                <p>
                                    This 
                                     
                                    <span>option</span> 
                                    only requires that other
                                    environments in the replication group
                                    participate in the vote. There is no
                                    requirement that
                                    <span class="emphasis"><em>all</em></span> such
                                    environments participate. In other
                                    words, if an environment
                                    starts up, it can call for an
                                    election, and select a master, even 
                                    if all other environment have not yet
                                    joined the  replication group.
                            </p>
              </li>
              <li>
                <p>
                                    It only requires a simple majority of
                                    participating environments to elect a master. 
                                    This is always true of elections held using the Replication Manager.
                            </p>
              </li>
              <li>
                <p>
                                    As always, the environment participating in the election with the most
                                    up-to-date log files is selected as
                                    master. If an environment with more recent log files
                                    has not yet joined the replication
                                    group, it may not become the master.
                            </p>
              </li>
            </ul>
          </div>
          <p>
                    Any one of these points may be enough to cause a
                    less-than-optimum environment to be selected as master.
                    Therefore, to give you a better degree of control over
                    which environment becomes a master at application startup,
                    the Replication Manager offers the following start-up
                                     
                                    <span>options:</span> 
            </p>
          <div class="informaltable">
            <table border="1" width="80%">
              <colgroup>
                <col />
                <col />
              </colgroup>
              <thead>
                <tr>
                  <th>Option</th>
                  <th>Description</th>
                </tr>
              </thead>
              <tbody>
                <tr>
                  <td>
                    <code class="literal">ReplicationManagerStartPolicy. REP_MASTER</code>
                  </td>
                  <td>
                   <p>
                   The application starts up and declares the environment to be a master
                   without calling for an election. It is an error for more
                   than one environment to start up using this flag, or for
                   an environment
                   to use this flag when a master already exists. 
                   </p>
                   <p>
                           Note that no replication group should
                           <span class="emphasis"><em>ever</em></span> operate with more than
                           one master.
                   </p>
                   <p>
                           In the event that a environment attempts to become a
                           master when a master already exists, the
                           replication code will resolve the problem by
                           holding an election. Note, however, that there
                           is always a possibility of data loss in the face
                           of duplicate masters, because once a master is
                           selected, the environment that loses the election will
                           have to roll back any transactions committed
                           until it is in sync with the "real" master.
                   </p>

                  </td>
                </tr>
                <tr>
                  <td>
                    <code class="literal">ReplicationManagerStartPolicy. REP_CLIENT</code>
                  </td>
                  <td>
                          <p>
                                  The application starts up and declares
                                  the environment to be a replica without calling for
                                  an election. Note that the environment
                                  can still become a master if a subsequent
                                  application starts up, calls for an
                                  election, and this environment is elected
                                  master.
                          </p>
                  </td>
                </tr>
                <tr>
                  <td>
                    <code class="literal">ReplicationManagerStartPolicy. REP_ELECTION</code>
                  </td>
                  <td>
                          <p>
                          As described above, the application starts up,
                          looks for a master, and if one is not found calls
                          for an election.
                          </p>
                  </td>
                </tr>
              </tbody>
            </table>
          </div>
        </div>
        <div class="sect2" lang="en" xml:lang="en">
          <div class="titlepage">
            <div>
              <div>
                <h3 class="title"><a id="thread_count"></a>Selecting the Number of Threads</h3>
              </div>
            </div>
          </div>
          <p>
                    Under the hood, the Replication Manager is threaded and you can
                    control the number of threads used to process messages received from
                    other replicas. The threads that the Replication Manager uses are:
            </p>
          <div class="itemizedlist">
            <ul type="disc">
              <li>
                <p>
                                    Incoming message thread. This thread
                                    receives messages from the site's
                                    socket and passes those messages to
                                    message processing threads (see below)
                                    for handling.
                            </p>
              </li>
              <li>
                <p>
                                    Outgoing message thread. Outgoing
                                    messages are sent from whatever thread
                                    performed a write to the database(s).
                                    That is, the thread that called, for
                                    example, 
                                    
                                    
                                    <code class="methodname">Database.put()</code>
                                    is the thread that writes replication messages
                                    about that fact to the socket. 
                            </p>
                <p>
                                    Note that if this write activity would
                                    cause the thread to be blocked due to
                                    some condition on the socket, the Replication Manager
                                    will hand the outgoing message to the
                                    incoming message thread, and it will
                                    then write the message to the socket.
                                    This prevents your database write
                                    threads from blocking due to abnormal
                                    network I/O conditions.
                            </p>
              </li>
              <li>
                <p>
                                    Message processing threads are
                                    responsible for parsing and then
                                    responding to incoming replication
                                    messages. Typically, a response will
                                    include write activity to your
                                    database(s), so these threads can be
                                    busy performing disk I/O.
                            </p>
              </li>
            </ul>
          </div>
          <p>
                    Of these threads, the only ones that you have any
                    configuration control over are the message processing
                    threads. In this case, you can determine how many
                    of these threads you want to run.
            </p>
          <p>
                    It is always a bit of an art to decide on a thread count,
                    but the short answer is you probably do not need more
                    than three threads here, and it is likely that one will
                    suffice. That said, the best thing to do is set your
                    thread count to a fairly low number and then increase
                    it if it appears that your application will benefit
                    from the additional threads.
            </p>
        </div>
      </div>
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